Field of the Invention
The present invention relates to an apparatus for the chemical-mechanical polishing of wafers, having a rotating disk provided with a polishing body, a supply device for a polishing fluid and a holding device for the wafer.
Chemical-mechanical polishing (CMP) was employed for the first time on a relatively large scale in the production of 16-megabit DRAMs and has since proved to be a method of ever-increasing importance. Thus, for example, it is employed for the production of trenches, trench insulations and metal tracks as well as for the planarization of intermetallic dielectrics (IMD). One example of trench insulation is so-called shallow trench insulation (STI). It was shown that, in all of the above-mentioned fields of use of chemical-mechanical polishing, problems arise in so far as sharp fluctuations in the removal of material frequently occur over a wafer and from wafer to wafer.
There are also similar problems in the production of SOI wafers (SOI=silicon on insulator) through the use of the wafer bonding method. In that method, two wafers, which are composed of silicon and which each have a silicon dioxide layer on their surface, are "bonded" to one another through those silicon dioxide layers, so as to result, as a whole, in a semiconductor body which has an oxide layer in the middle that is composed of the two silicon dioxide layers. One of the wafers is then ground down, in order to finally obtain a thin silicon layer on the oxide layer. That grinding process is not sufficiently uniform per se in order to ensure a sufficiently small fluctuation in the thickness of the thin silicon layer.
The so-called PACE (Plasma-Assisted Chemical Etching) method has recently been employed in order to increase the uniformity of the grinding process (see "Microelectronic Engineering", Volume 22, page 301, 1993). In that method, first the wafer thickness remaining after a first coarse grinding step is measured in profile over the wafer. A relatively small plasma etcher, which has a diameter of between 3 and 30 mm and which is controlled by a computer through the use of measured layer thickness data, is then led over the wafer, so that the desired ultimate layer thickness can be achieved everywhere on the wafer. Thickness fluctuations which are below 10 nm can be achieved through the use of that procedure.
In conventional CMP, a rotating disk-shaped polishing body having a polishing cloth or "pad" is used, in order to polish a wafer which is placed in the region of a radius of the polishing body in the same plane as the surface of the latter.
.mu.-chemical-mechanical polishing (.mu.CMP) also uses the above-mentioned computer-controlled principle: after a first conventional CMP step, in which most of the material to be taken off is removed rapidly, the remaining layer thickness is measured over the wafer. For that purpose, special measuring zones are provided in each chip on the wafer, which are traversed by an automatic measuring instrument. If it is possible to measure all locations over the wafer, then specific predetermined points can also simply be selected. In that case, the measuring zones or the predetermined points must be placed so closely to one another that the layer thickness between the measuring zones or points is defined essentially by interpolation between the measuring zones or points. The wafer is then traversed by a special .mu.CMP polishing apparatus which only ever touches a relatively small region of the wafer momentarily.
In that case, at least three parameters are appropriate for varying the amount of material removed by polishing in conformity with the measurement data for the layer thickness: the rotational speed of the polishing body, the contact pressure of the polishing body on the wafer and the traveling speed of the polishing body relative to the surface of the wafer.
With regard to a round wafer, it is possible, for example, to use a spiral path of the polishing body. In that case, the polishing body is applied in the middle of the wafer and is then led spirally as far as the edge of the wafer, with the paths in each case overlapping one another to a greater or lesser extent. A spiral path of that type can be executed particularly easily. The holding device for the wafer or the wafer chuck rotates slowly, while the polishing body is led outward from the mid-point of the wafer in a linear movement. Alternatively, it is also conceivable to traverse the wafer in linear strips. It is also possible to employ a plurality of polishing bodies simultaneously on a wafer, in order to thereby shorten the machining time. In that case, the rotational speed and/or contact pressure and/or traveling speed may be set individually for each polishing body. At the same time, change in the rotational speed could possibly be advantageous for varying the removal of material.
Previous .mu.CMP apparatuses use polishing bodies, the axes of which run perpendicularly to the wafer surface in the same way as in the CMP apparatuses that have been known for some time. In other words, all of the previous apparatuses for the chemical-mechanical polishing of wafers have polishing bodies, the axis of rotation of which is led perpendicularly to the wafer surface. It is not beneficial to use the previous configuration, having an axis of rotation of the polishing body perpendicular to the wafer surface, and to merely employ a smaller polishing body, in order to grind a small part of the wafer surface.
That is because, during a polishing process, the surface of a polishing cloth or pad of the polishing body, which cloth or pad is composed of PU foam (PU=polyurethane) or a textile material, is not exposed so as to be sprinkled with a polishing fluid or "slurry".
For the same reason, it is difficult, during polishing, to treat the polishing cloth specially through the use of a grinding body for roughening purposes, since that is likewise possible only when the surface of the polishing cloth is accessible. So-called "pad conditioning" is therefore difficult to carry out.
Finally, a W-shaped material removal profile is obtained when a rotating grinding disk, with an axis perpendicular to the wafer surface, is drawn over the surface of the wafer, with the profile having steep flanks or lateral edges. The W-shape is attributable to the fact that the period of action between the polishing cloth and the wafer is short at the edge of the disk, while in the middle of the disk, the theoretical rotational speed "0" prevails. Such a W-shaped material removal profile is somewhat unsuitable for achieving uniform material removal. If, for example, two trenches having a W-shaped material removal profile of that type run parallel to one another, the steep flanks or lateral edges will cause any error in the relative position of the trenches to one another to result in sharp fluctuations in the removal of material in the overlap region.